Various endothermic and exothermic chemical reactions require effective heat transfer to maintain the reaction, especially in a commercial process unit, such as in a chemical plant or petroleum refinery where great amounts of heat need to be transferred. One such commercial chemical process that requires effective heat transfer is the Fischer-Tropsch synthesis process. Synthesis gas, a preferred feedstream to the reactor of this invention, is primarily comprised of carbon monoxide and hydrogen which can be produced from a range of carbonaceous feedstocks such as natural gas, coal, petroleum coke, heavy oils, biomass, landfill gas, biogas and municipal waste. Synthesis gas can be converted to a wide range of hydrocarbons such as methanol, mixed alcohols, olefins, paraffinic hydrocarbons and mixtures thereof. These reactions are generally referred to as Fischer-Tropsch and related synthesis. These materials are useful for production of a wide range of chemical and fuel products.
The Fischer-Tropsch synthesis reaction is a good reaction to demonstrate the present invention. This reaction is highly exothermic and therefore the synthesis reactor must be designed to conduct the reaction and effectively remove the heat of reaction as it is generated to control the reaction temperature. Additionally, the Fischer-Tropsch reaction is diffusion limited when the catalyst particle size is greater than about 300 microns. The use of larger particles leads to diminished productivity and higher methane selectivity, thereby requiring higher catalyst volumes and diminished production of the higher valued C8+ hydrocarbons. Tubular reactors generally must utilize catalyst particle sizes greater than 0.75 mm in order to limit pressure drop. Using a pillow panel design allows use of catalyst particle sizes in the range of about 200-600 microns.
Various types of reactors have been employed for Fischer-Tropsch and related synthesis reactions. Many of these reactors are modified heat exchangers or incorporate heat exchanger elements in the reactor designs. The reactor capacity may be limited due to heat and/or mass transfer problems. The first Fischer-Tropsch reactors used in Germany in 1936 were the lamella plate design, wherein a reactor shell was a rectangular tank made of iron plate that, on account of the normal pressure operation, was of very simple design. However, the internal section of the reactor was of a much more complex design. The reaction chamber was occupied by a lamella bundle consisting of 2 mm thick vertical iron plates spaced 7 mm apart to transfer the heat of reaction to numerous horizontal cooling tubes in which pressurized water was circulating. The catalyst, a fine granulate or extrudate, was located in the gaps between the vertical lamellas.
Lamella reactors had several drawbacks, for example they were mechanically complex and had poor heat transfer. Although the catalyst was well distributed and close to a heat transfer surface or plates, however, the plates were just extended surfaces from the panels and the heat transfer was not very effective. The poor heat removal limited the reactor capacity and led to local hot spots which caused catalyst damage.
After the lamella plate reactor, a special type of multi-tubular reactor was developed wherein the catalyst was placed in the space between two concentric tubes. Heat removal was accomplished via pressurized water that surrounded the outer tube and also had access to the inner tube via a short connecting pipe. Therefore, heat transfer was improved compared to that of the lamella reactor, but the capacity was still relatively low and the reactor was mechanically complex.
After World War II the South African company Sasol employed an improved tubular reactor known as the ARGE high capacity reactor. ARGE reactors were designed like a vertical shell and tube heat exchanger with boiling water in the shell and catalyst packed in the tubes. These reactors were a significant improvement over the lamella plate reactors, or the twin tube reactors, in terms of heat transfer and productivity. However, the tubes were long which limits catalyst particle size due to pressure drop and the large number of tubes requires a long time to load and unload the catalyst.
U.S. Pat. No. 7,084,180 describes a micro channel reactor for Fischer-Tropsch synthesis. The micro channel reactor operates at very high productivities on the order of 10-20 times more than the productivity of ARGE reactors. Such reactors, in spite of their very high performance, are difficult to construct and do not scale up well to large single train capacity.
In addition to the range of fixed bed reactors that have been employed for Fischer-Tropsch and related synthesis, there have been moving bed reactors such as fluidized bed reactors and slurry bubble column reactors. Such reactors have difficulty with catalyst attrition, catalyst filtration and scale up. The slurry reactor utilizes smaller catalyst particles which eliminate the product debits associated with larger particles. However the slurry reactor is both costly and difficult to scale
Although most of the above mentioned Fischer-Tropsch reactors have met with some degree of success there is need in the art for a synthesis reactor that is readily scalable, that provides excellent heat transfer characteristics at high productivities while utilizing optimized catalyst materials and that can be constructed easily in moderate to large single train capacity. The pillow panel design of the present invention offers the advantages in using smaller catalyst particles in a reactor configuration which scales directly from the small pilot scale (<1 bbl/day) to the larger sizes (>1000 bbl/day).